Method of optimizing channel characteristics using multiple masks to form laterally crystallized ELA poly-Si films

ABSTRACT

A method is provided to optimize the channel characteristics of thin film transistors (TFTs) on polysilicon films. The method is well suited to the production of TFTs for use as drivers on liquid crystal display devices. The method is also well suited to the production of other devices using polysilicon films. Regions of polycrystalline silicon can be formed with different predominant crystal orientations. These crystal orientations can be selected to match the desired TFT channel orientations for different areas of the device. The crystal orientations are selected by selecting different mask patterns for each of the desired crystal orientation. The mask patterns are used in connection with lateral crystallization ELA processes to crystallize deposited amorphous silicon films.

CROSS-REFERENCES

[0001] The subject matter of this application is related to theapplication entitled Method of Optimizing Channel Characteristics usingLaterally-Crystallized ELA Poly-Si Films by inventors Apostolos Voutsas,John W. Hartzell and Yukihiko Nakata filed on the same date as thisapplication (Attorney Docket No. SLA 0513).

[0002] The subject matter of this application is also related to theapplication entitled Mask Pattern Design to Improve Quality Uniformityin Lateral Laser Crystallized Poly-Si films by inventor ApostolosVoutsas filed on the same date as this application (Attorney Docket No.SLA 0512).

[0003] All of these applications, which are not admitted to be prior artwith respect to the present invention by their mention here, areincorporated herein by this reference.

BACKGROUND OF THE INVENTION

[0004] This invention relates generally to semiconductor technology andmore particularly to a method of forming polycrystalline silicon withinan amorphous silicon film.

[0005] Polycrystalline silicon thin film transistors (TFTs) can be usedin a variety of microelectronics applications, especially active matrixliquid crystal displays (LCDs).

[0006] Thin film transistors (TFTs) used in liquid crystal displays(LCDs) or flat panel displays of the active matrix display type arefabricated on silicon films deposited on a transparent substrate. Themost widely used substrate is glass. Amorphous silicon is readilydeposited on glass. Amorphous silicon limits the quality of TFTs thatcan be formed. If driver circuits and other components are to be formedon the display panel, as well as switches associated with each pixel,crystalline silicon is preferred.

[0007] Silicon is often referred to as either amorphous or crystalline,including single crystal silicon. The term crystalline silicon can referto either single crystal silicon, polycrystalline silicon, or in somecases materials with significant quantities of micro-crystal structures.For many application, single crystal material is most desirable. But,single crystal silicon is not readily producible. Amorphous silicon canbe crystallized to form crystalline silicon by solid-phasecrystallization. Solid-phase crystallization is carried out by hightemperature annealing. But, glass substrates cannot withstand thetemperatures necessary to melt and crystallize silicon. Quartzsubstrates can withstand high temperature annealing, but quartzsubstrates are too expensive for most LCD applications.

[0008] Because glass deforms when exposed to temperatures above 600° C.,low-temperature crystallization (preferably below 550° C.) is used forsolid-phase processing of silicon on glass. The low-temperature processrequires long anneal times (at least several hours). Such processing isinefficient and yields polycrystalline silicon TFTs that have relativelylow field effect mobility and poor transfer characteristics.Polycrystalline silicon produced by solid-phase crystallization ofas-deposited amorphous silicon on glass suffers due to small crystalsize and a high density of intragrain defects in the crystallinestructure.

[0009] Excimer laser annealing (ELA) has been actively investigated asan alternative to low-temperature solid-phase crystallization ofamorphous silicon on glass. In excimer laser annealing, a high-energypulsed laser directs laser radiation at selected regions of the targetfilm, exposing the silicon to very high temperatures for shortdurations. Typically, each laser pulse covers only a small area (severalmillimeters in diameter) and the substrate or laser is stepped throughan exposure pattern of overlapping exposures, as is known in the art.

[0010] Lateral crystallization by excimer laser annealing (LC-ELA) isone method that has been used to form high quality polycrystalline filmshaving large and uniform grains. LC-ELA also provides controlled grainboundary location.

[0011] According to one method of conducting LC-ELA, an initiallyamorphous silicon film is irradiated by a very narrow laser beamlet,typically 3-5 micrometers wide. Passing a laser beam through a mask thathas slits forms the beamlet, which is projected onto the surface of thesilicon film.

[0012] The beamlet crystallizes the amorphous silicon in its vicinityforming one or more crystals. The crystals grow within the areairradiated by the beamlet. The crystals grow primarily inward from edgesof the irradiated area toward the center. The distance the crystalgrows, which is also referred to as the lateral growth length, is afunction of the amorphous silicon film thickness and the substratetemperature. Typical lateral growth lengths for 50 nm films isapproximately 1.2 micrometers. After an initial beamlet has crystallizeda portion of the amorphous silicon, a second beamlet is directed at thesilicon film at a location less than half the lateral growth length fromthe previous beamlet. Moving either the laser, along with its associatedoptics, or by moving the silicon substrate, typically using a stepper,changes the location of the beamlet. Stepping a small amount at a timeand irradiating the silicon film causes crystal grains to grow laterallyfrom the crystal seeds of the poly-Si material formed in the previousstep. This achieves lateral pulling of the crystals in a manner similarto zone-melting-crystallization (ZMR) methods or other similarprocesses.

[0013] As a result of this lateral growth, the crystals produced tend toattain high quality along the direction of the advancing beamlets, alsoreferred to as the “pulling direction.” However, the elongated crystalgrains produced are separated by grain boundaries that run approximatelyparallel to the long grain axes, which are generally perpendicular tothe length of the narrow beamlet.

[0014] When this poly-Si material is used to fabricate electronicdevices, the total resistance to carrier transport is affected by thecombination of barriers that a carrier has to cross as it travels underthe influence of a given potential. Due to the additional number ofgrain boundaries that are crossed when the carrier travels in adirection perpendicular to the long grain axes of the poly-Si material,the carrier will experience higher resistance as compared to the carriertraveling parallel to the long grain axes. Therefore, the performance ofTFTs fabricated on poly-Si films formed using LC-ELA will depend uponthe orientation of the TFT channel relative to the long grain axes,which corresponds to the main growth direction. Typically, TFTperformance varies by a factor of between 2 and 4 as a function oforientation relative to the main growth direction.

[0015] This difference in performance is undesirable from the point ofview that as LCD resolution increases, or as panel size decreases, sizelimitations make it more desirable to have column drivers and rowdrivers oriented at ninety degrees relative to each other. Potentiallyresulting in one set of drivers having significantly differentcharacteristics relative to the other.

SUMMARY OF THE INVENTION

[0016] Accordingly, a method of forming polycrystalline regions on asubstrate is provided. A first mask pattern is selected. A laser beam isdirected through the first mask pattern to irradiate the substrate overan initial region on the substrate. The region is annealed using alateral crystallization process. A second mask pattern is selected. Thelaser beam is directed through the second mask pattern to irradiate thesubstrate over a second region on the substrate. The region is annealedusing a lateral crystallization process. If the first and second maskpattern have different orientations, the first region will have adifferent crystal orientation than the second region followingannealing.

[0017] The method of the present invention is well suited for producingdevices using polycrystalline silicon. One application would beproducing driver circuits for LCDs. In which case, an LCD substrate,which can be composed of quartz, glass, plastic or other suitabletransparent material, is used. An amorphous semiconductor material isdeposited on the LCD substrate to form a thin layer of amorphoussilicon. Preferably the semiconductor material will be silicon. A firstregion of the amorphous silicon is annealed using a first mask patternin connection with a lateral crystallization ELA process to form a firstpolycrystalline region having elongated grain structures with a firstcrystal orientation. A second region of the amorphous silicon isannealed using a second mask pattern in connection with a lateralcrystallization ELA process to form a second polycrystalline regionhaving elongated grain structures with a second crystal orientation. Thesecond crystal orientation is different from the first crystalorientation, and preferably the crystal orientations are substantiallyninety degrees with respect to each other.

[0018] For certain applications it is desirable to form thin filmtransistors (TFTs) using the polycrystalline material formed by laserannealing. In a preferred embodiment of the present method, TFTs havinga first channel orientation are formed over the region with the firstcrystal orientation. The channel orientation is preferably substantiallyparallel to the crystal orientation, whereby the fewest number ofcrystal grain boundaries are crossed by the channel. TFTs having asecond channel orientation formed over the region with the secondcrystal orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic cross-sectional view showing an ELAapparatus used in connection with the present method.

[0020]FIG. 2 shows mask patterns.

[0021]FIG. 3 illustrates a step in the process of lateralcrystallization using ELA.

[0022]FIG. 4 illustrates a step in the process of lateralcrystallization using ELA.

[0023]FIG. 5 illustrates a step in the process of lateralcrystallization using ELA.

[0024]FIG. 6 is a flowchart diagram of an embodiment of the presentinvention.

[0025]FIG. 7 illustrates the formation of a substrate with multipleregions of different crystal orientation.

[0026]FIG. 8 illustrates the formation of TFTs with channels aligned tothe crystal orientation to optimize performance.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring to FIG. 1 a lateral crystallization excimer laserannealing (LC-ELA) apparatus 10 is shown. LC-ELA apparatus 10 has alaser source 12. Laser source 12 may include a laser (not shown) alongwith optics, including mirrors and lens, which shape a laser beam 14(shown by dotted lines) and direct it toward a substrate 16, which issupported by a stage 17. The laser beam 14 passes through a mask 18supported by a mask holder 20. The laser beam 14 preferably has anoutput energy in the range of 0.8 to 1 Joule when the mask 18 is 50mm×50 mm. Currently available commercial lasers such as Lambda Steel1000 can achieve this output. As the power of available lasersincreases, the energy of the laser beam 14 will be able to be higher,and the mask size will be able to increase as well. After passingthrough the mask 18, the laser beam 14 passes through demagnificationoptics 22 (shown schematically). The demagnification optics 22 reducethe size of the laser beam reducing the size of any image produced afterpassing through the mask 18, and simultaneously increasing the intensityof the optical energy striking the substrate 16 at a desired location24. The demagnification is typically on the order of between 3x and 7xreduction, preferably a 5x reduction, in image size. For a 5x reductionthe image of the mask 18 striking the surface at the location 24 has 25times less total area than the mask, correspondingly increasing theenergy density of the laser beam 14 at the location 24.

[0028] The stage 17 is preferably a precision x-y stage that canaccurately position the substrate 16 under the beam 14. The stage 17 ispreferably capable of motion along the z-axis, enabling it to move upand down to assist in focusing or defocusing the image of the mask 18produced by the laser beam 14 at the location 24.

[0029] The mask holder 20 is preferably capable of x-y movement. Asshown, the mask holder is holding two mask patterns. The two maskpatterns could be formed on a single mask, or could be two separatemasks. It is entirely possible, and within the scope of the presentinvention, to have more than two mask patterns. The mask holders x-ymovement can be used to position the desired mask pattern under thelaser beam 14. The mask patterns can be laid out linearly as shown. Orin the case of more than two mask patterns, the mask patterns can bearranged in a two dimensional array.

[0030]FIG. 2 shows a mask 18 having a first mask pattern 26 and a secondmask pattern 27. The first mask pattern 26 comprises a first set ofslits 29 with a first slit spacing 30. The second mask pattern 27comprises a second set of slits 31 with a second slit spacing 32. In apreferred embodiment, the second mask pattern 27 corresponds to thefirst mask pattern 26 rotated by a desired angle. In a preferredembodiment, the desired angle is substantially ninety degrees. As shownboth mask patterns are formed on a single mask, an alternativeembodiment would use multiple masks to provide multiple patterns.

[0031]FIGS. 3 through 5 show the sequence of lateral crystallizationemployed as a portion of the present method. A region 34 of amorphous orpolycrystalline silicon overlies the substrate. The rectangular area 36corresponds to an image of one of the slits 30 projected on thesubstrate 16. The dashed line 38 corresponds to the centerline of theimage of the opening on the substrate.

[0032]FIG. 3 shows the region 34 just prior to crystallization. A laserpulse is directed at the rectangular area 36 causing the amorphoussilicon to crystallize. After each pulse the image of the opening isadvanced by an amount not greater than half the lateral crystal growthdistance. A subsequent pulse is then directed at the new area. Byadvancing the image of the slits 30 a small distance the crystalsproduced by preceding steps act as seed crystals for subsequentcrystallization of adjacent material. By repeating the process ofadvancing the image of the slits and firing short pulses the crystal iseffectively pulled in the direction of the slits movement.

[0033]FIG. 4 shows the region 34 after several pulses. As is clearlyshown, the area 40 that has already been treated has formed elongatedcrystals that have grown in a direction substantially perpendicular tothe length of the slit. Substantially perpendicular means that amajority of lines formed by crystal boundaries 42 could be extended tointersect with dashed line 38.

[0034]FIG. 5 shows the region 34 after several additional pulsesfollowing FIG. 4. The crystals have continued to grow in the directionof the slits' movement to form a polycrystalline region. The slits willpreferably continue to advance a distance substantially equal to adistance on the substrate corresponding to the slit spacing 32. Eachslit will preferably advance until it reaches the edge of apolycrystalline region formed by the slit immediately preceding it.

[0035] Referring now to FIG. 6, a flow chart of the steps of the methodof the present invention is shown. Step 110 selects a first maskpattern.

[0036] Step 120 performs lateral crystallization using excimer laserannealing (ELA) to produce a polycrystalline region having a firstcrystal orientation. A laser beam is used to project an image of themask onto the substrate. The laser beam energy is sufficient to causeamorphous silicon to crystallize. As discussed above a sequence of laserpulses can be used to crystallize a region with a first crystalorientation.

[0037] Step 130 selects a second mask pattern. This second mask patternis preferably identical to the first mask pattern rotated to a differentangle. Preferably the different angle is substantially perpendicular tothe first pattern.

[0038] Step 140 performs lateral crystallization ELA to produce apolycrystalline region having a second crystal orientation. The secondcrystal orientation is preferably substantially perpendicular to thefirst crystal orientation.

[0039] The steps of selecting the first or second mask pattern (steps110 and 130) can be accomplished using a mask holder capable ofrepositioning the mask to orient a desired pattern under the laser beam14. The mask patterns can be on a single mask, or each mask pattern canbe located on different individual masks. Although, we have describedthe method of the present invention in terms of using two mask patterns,in some applications it may be desirable to use three or more patterns.If additional patterns are used the mask holder will preferably becapable of selecting from multiple mask patterns. Preferably, the maskholder selects masks by using x-y motion to reposition the mask holderunder the laser beam 14.

[0040] In performance of the method, if multiple regions of the sameorientation are desired, it is preferable to produce all of the regionswith the first crystal orientation prior to changing the mask andproducing regions of the second crystal orientation. Multiple regionswith the same orientation are preferred when producing multiple deviceson a single substrate.

[0041]FIG. 7 shows the substrate 16 with two display regions 210 and220. Each display region corresponds to the location of a final LCD orother display device. The first mask is selected. Then the image 222 ofthe first mask is projected at a first starting position 224.

[0042] In an embodiment of the present method, the image 222 is movedone step at a time by moving the mask stage. At each step a laser pulsecrystallizes a portion of the silicon material. Once the image 222 hasmoved a distance corresponding to the slit spacing, the substrate ismoved to position the image 222 over an adjacent position 226. The maskstage is then moved to crystallize the underlying region. By repeatingthis process across the substrate, a line of polycrystalline materialhaving predominantly a first crystal orientation is formed. The image222 is repositioned at a position corresponding the start of theadjacent uncrystallized region. The process is repeated until a region230 is formed having predominantly a first crystal orientation. As shownthis orientation is horizontal. After a first region 230 is formed,repeating the process discussed above can produce a second region 240having the same general crystal orientation as the first region 230.

[0043] In a preferred embodiment, once regions of a first crystalorientation have been produced, the second mask is selected. The secondmask preferably has a pattern that is substantially perpendicular to thefirst mask pattern. The process is then repeated to produce regions witha second crystal orientation. Preferably, the second crystal orientationwill be substantially perpendicular to the first crystal orientation. Athird region 250 is formed by positioning the second mask image 245 overanother starting point and processing the region as discussed aboveuntil the region 250 has been crystallized. A fourth region 260 couldthen be 10 crystallized to have the same orientation as the third region250.

[0044] In this manner, multiple regions can be crystallized with two ormore crystal orientations. The order of crystallization is not criticalto the present invention.

[0045] Once the substrate 16 has been processed to form regions with thedesired crystal orientation, device elements are formed on the substrateas illustrated in FIG. 8. FIG. 8 is for illustration purposes, and aswith the other drawings, is not drawn to scale. The substrate 16 has afirst polycrystalline region 330 and a second polycrystalline region 340with the same crystal orientation. A first set of TFTs 345 have been 20formed within polycrystalline regions 330 and 340. First set of TFTs 345have channels 347 oriented to match the crystal orientation of theunderlying regions 330 and 340. As shown in the figure, both the crystalorientation of regions 330 and 340, and the channels 347 are horizontal.Third polycrystalline region 350 and fourth polycrystalline region 360are shown having a crystal orientation substantially perpendicular tothe crystal orientation of regions 330 and 340. A second set of TFTs 365having channels 367 are substantially perpendicular to the first set ofTFTs 345 and channels 347, and substantially parallel to the crystalorientation of the underlying regions 350 and 360.

[0046] Since FIG. 8 illustrates a display device, pixel regions 370 areshown. The pixel regions 370 can have the same underlying crystalorientation as either the regions under the first set of TFTs 345, alsoreferred to as row drivers, or the second set of TFTs 360, also referredto as the column drivers. As shown in FIG. 8, the pixel region ismatched to the column drivers. If the substrate shown in FIG. 7 wereused, the pixel region would match the row drivers. For someapplications, it may not be necessary to crystallize the entiresubstrate. Some regions may not need to be crystallized including, butnot limited to the pixel regions.

[0047] Although the present method is well suited to producing displaydevices, it is also suited to other types of device produced using apolycrystalline material produced on an underlying substrate. Inaddition to row and column drivers, other circuitry unrelated todisplays can be produced.

[0048] The terms perpendicular and parallel should not be construednarrowly to limit the scope of the present method, especially inreference to crystal orientation. The terms substantially perpendicularand substantially parallel should be construed broadly. A broaderdefinition of these term parallel is therefore provided. If a feature,or structure, is said to be parallel to the crystal orientation, thestructure crosses the fewest crystal grain boundaries in the relevantdirection.

[0049] Several embodiments of the method of the present invention havebeen described. Variations on these embodiments will be readilyascertainable by one of ordinary skill in the art. Therefore, thedescription here is for illustration purposes only and should not beused to narrow the scope of the invention, which is defined by theclaims as interpreted by the rules of patent claim construction.

What is claimed is:
 1. A method of forming polycrystalline regions on asubstrate comprising the steps of: a) selecting a first mask pattern; b)directing a laser beam through the first mask pattern to irradiate thesubstrate over an initial region on the substrate; c) annealing theinitial region using a lateral crystallization process; d) selecting asecond mask pattern; e) directing the laser beam through the second maskpattern to irradiate the substrate over a second region on thesubstrate; and f) annealing the second region using a lateralcrystallization process.
 2. The method of claim 1, wherein the firstmask pattern is a plurality of parallel slits at a first orientation. 3.The method of claim 2, wherein the second mask pattern is a plurality ofparallel slits at a second orientation.
 4. The method of claim 3,wherein the first orientation and the second orientation areapproximately ninety degrees relative to each other.
 5. The method ofclaim 1, wherein the mask is mounted to a mask holder, which isselectable to allow a mask pattern to be selected.
 6. A method ofprocessing a substrate comprising the steps of: a) depositing amorphoussilicon on a substrate; b) annealing a first region on the substrateusing a first mask pattern in connection with a lateral crystallizationELA process to form a first polycrystalline region having elongatedgrain structures with a first orientation; c) annealing a second regionon the substrate using a second mask pattern in connection with alateral crystallization ELA process to form a second polycrystallineregion having elongated grain structures with a second orientation,which is different from the first orientation; d) forming a first TFThaving a channel oriented substantially parallel to the elongated grainstructures of the first polycrystalline region; and e) forming a secondTFT having a channel oriented substantially parallel to the elongatedgrain structures of the second polycrystalline region.
 7. The method ofclaim 6, wherein the substrate is a transparent material.
 8. The methodof claim 6, wherein the substrate is quartz, glass, or plastic.
 9. Themethod of claim 6, wherein the second orientation is substantiallyperpendicular to the first orientation.
 10. The method of claim 6,wherein the first mask pattern and the second mask pattern are formed onthe same mask.
 11. The method of claim 6, wherein the mask is mounted toa mask holder, which is capable of selecting from a plurality of maskpatterns.
 12. The method of claim 6, wherein the mask is mounted to amask holder, which is capable of selecting from a plurality of masks.13. The method of claim 6, wherein the first mask pattern is formed on afirst mask.
 14. The method of claim 6, wherein the second mask patternis formed on a second mask.
 15. A method of processing an LCD substratecomprising the steps of: a) depositing amorphous silicon on a substrate;b) selecting a first mask pattern; c) forming row drivers by annealing afirst plurality of regions on the substrate using a lateralcrystallization ELA process to form polycrystalline regions havingelongated grain structures with a first orientation and forming a firstplurality of TFT structures having channels oriented substantiallyparallel to the elongated grain structures with the first orientation;d) selecting a second mask pattern; and e) forming column drivers byannealing a second plurality of regions on the substrate using a lateralcrystallization ELA process to form polycrystalline regions havingelongated grain structures with a second orientation, which is differentfrom the first orientation and forming a second plurality of TFTstructures having a channel oriented substantially parallel to theelongated grain structures of the second polycrystalline region.
 16. Themethod of claim 15, wherein the second orientation is substantiallyperpendicular to the first orientation.
 17. The method of claim 15,further comprising forming a pixel region.
 18. The method of claim 17,wherein the pixel region is formed by annealing a region on thesubstrate using a lateral crystallization ELA process to formpolycrystalline regions having elongated grain structures with the firstorientation.
 19. The method of claim 17, wherein the pixel region isformed by annealing a region on the substrate using a lateralcrystallization ELA process to form polycrystalline regions havingelongated grain structures with the second orientation.